Upgrade wrapper-validation-action to gradle/actions/wrapper-validation@v4

The old gradle/wrapper-validation-action@v1 was silently failing on all
PR runs (works on push-to-main but not on PR merge commits). The modern
replacement is gradle/actions/wrapper-validation which is actively
maintained by Gradle.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>